I've been doing artificial harmonics for a while now and I've found they come out much higher when I pluck the string (using my thumb to sound the a.h.) just above the centre of my bridge pickup. I've also found them harder to do with a soft pick so make sure you have a nice firm pick, and de-tuning your strings so they are a little slacker helps bring them out quite nicely. I've always admired Dimebag's artificial harmonics because he makes them absolutely *squeal* and he adjusts the settings on his amp so that bass and treble are turned all the way up and mid turned all the way down, for that real chunky sound and amazing pinched harmonics.